Pictures of Rhizaria:                                          

Characteristics of Rhizaria:                                

The Rhizaria (meaning with roots, because many of their species possess microscopic structures resembling roots) is a kingdom of primarily microscopic and aquatic organisms, although some species are terrestrial and inhabit soils. They generally go unnoticed in ecosystems, but in certain areas, such as marine coastlines or plankton-rich zones, they play a very important ecological role.

They are a group with highly varied lifestyles, generally unicellular and microscopic, but with some macroscopic species reaching several centimeters, featuring giant cells. It is a challenging group to define, but amoeboid forms dominate, with characteristic cell expansions formed by a specialized cytoskeleton. These structures often collect particles and other organisms which are introduced into the cell for its digestion. They typically feed on detritus or hunt small microorganisms, but some species are photosynthetic or parasitic on plants and other hosts. Many species form shells or calcareous or siliceous skeletons, sometimes highly elaborate, with chambers or morphologically complex structures characteristic of certain groups. These shells, upon the organism's death, accumulate on the seafloor and can form sediment layers kilometers thick.

This kingdom is related to the Chromalveolata kingdom, which includes many photosynthetic organisms, thus sharing a common ancestor with Rhizaria. This unicellular common ancestor was heterotrophic, meaning it fed on organic matter, but after merging with a red alga (phylum Rhodophyta), it became photosynthetic. Photosynthesis was retained in some major descendant lineages, such as in Chromalveolata, but was completely lost in the Rhizaria, which lack photosynthetic organelles but do possess genes derived from the red alga. However, there is a small group of species with photosynthetic capacity acquired through a new symbiosis with a green alga at the intracellular level. This group, after ingesting a unicellular green alga (phylum Chlorophyta), was able to keep it alive within its cell, retaining over millions of years of evolution only the machinery necessary for photosynthesis, so that the symbiotic alga is barely recognizable. Apart from this phenomenon of merging with a green alga, the rhizarians have a small group of species (genus Paulinella) that are also photosynthetic, but not due to a symbiosis with an alga, but with a photosynthetic bacterium from the Cyanobacteria group.

The following image shows the evolutionary relationships of the main Rhizaria lineages: